Conventional X-ray imaging systems installed in hospitals are classified into film radiography and digital radiography. In film radiography, a patient is irradiated with X-rays, and the X-rays that have passed through the patient are exposed to a film. In digital radiography, X-rays that have passed through a patient are converted into an electrical signal, and the electrical signal is detected as a digital value by using an A/D converter and stored in a memory.
An example of the current mainstream of the latter digital radiography is disclosed in Japanese Patent Laid-Open No. 5-224322. In this scheme, an X-ray image is formed on a photostimulable phosphor called an imaging plate (IP) using BaBr:Eu as a typical material. Then, the IP is scanned with a laser beam, and visible light from the IP is converted into an electrical signal, i.e., digitized by using, e.g., a photomultiplier.
According to a scheme disclosed in Japanese Patent Laid-Open No. 8-116044, a phosphor is irradiated with X-rays. Visible light emitted in proportion to the X-ray dose is converted into an electrical signal, i.e., digitized by a photosensor made of amorphous silicon. The typical materials of the phosphor are Gd2O2S:Tb and CsI:Tl. This device is called an FPD (Flat Panel Detector). Some FPDs use, as the material, Se or PbI2 that directly absorbs X-rays and converts them into an electrical signal instead of using the phosphor.
There is also a device which irradiates a primary phosphor with X-rays. Photoelectrons from the phosphor surface are accelerated and focused by an electron lens, and a phosphor image (X-ray image) on a secondary phosphor surface is converted into an electrical signal by a camera tube or CCD. This is a general scheme called an image intensifier (I.I.) and used for fluorography. It is a digital radiographic scheme capable of detecting an electrical signal as a digital value.
As described above, there exist a variety of devices for digitizing an X-ray image, and a demand for them is increasingly growing in recent years. When image data can be digitized, radiographic data can easily be recorded, displayed, printed, and stored. Hence, the need for digitization in the medical field is increasing.
In the recent medical field which is shifting from film radiography, i.e., so-called analog radiography to the above-described digital radiography, plain X-ray imaging is performed as the first step of X-ray imaging. For, e.g., a chest part, the imaging is called chest plain X-ray imaging, and X-ray imaging of the front (or side) of the chest part of a human body is done. To cover the whole chest part (upper part) of a human body, a radiography area of 14″×17″ size (35 cm×43 cm) or more and, more preferably, an area of 43 cm×43 cm or more is necessary. In the chest plain X-ray imaging, the FPD is a more promising digital radiography scheme than I.I. which has a problem of distortion of a peripheral image.
When plain radiography is executed as the first step of X-ray imaging, the doctor diagnostically reads the radiographed image. If a shadow is recognized, CT is generally executed as the second step of X-ray imaging. CT is performed to obtain a tomographic image of the part recognized by plain radiography. The radiation dose of CT is generally larger than that of plain radiography. For this reason, CT is executed only after plain radiography or equivalent tests except a few cases such as emergency.
In the CT, normally, X-rays from an X-ray tube are focused by a collimator, and a patient is irradiated with X-rays called a fan beam having a fan shape. Transmission light is detected by using X-ray detection elements which are arranged on the opposite side of the patient to detect the X-rays. The CT is executed while making the set of the X-ray tube and X-ray detection elements helically rotate around the radiographic part of the patient. Obtained image data is reconstructed to 3D image data by using a computer.
In the CT using the fan beam, X-ray detection elements arranged in a liner array direction or in several lines are used, and much time is required from the start to the end of radiography. For this reason, the burden on the patient who is fixed to restrict motion and let stand in a closed space called a gantry for a long time becomes heavy. There are also a problem of high power consumption and a problem of the life of the X-ray tube (a problem of replacement frequency). To solve these problems, Japanese Patent Laid-Open No. 4-343836 uses a large-area X-ray detection element including X-ray detection elements arranged two-dimensionally. This prior art also proposes a method of obtaining a CT image by executing helical scan while irradiating a patient with cone-beam X-rays.
In the chest plain X-ray imaging, internal information near the lung field of the upper body, including the gullet, trachea, pulmonary vessels, alveoli, heart, cardiac vessels, diaphragm, ribs, and collar bones, can be radiographed in one image by X-ray imaging of one cycle. Hence, the chest plain X-ray imaging is frequently used as a useful radiographic method to screen a lesion (morbid portion).
In the conventional chest plain X-ray imaging, a fluoroscopic image is observed because of its principle. Hence, if the morbid portion to be observed is located, e.g., behind a rib, cardiac vessel, or diaphragm, the fluoroscopic images overlap, and the shadow of the morbid portion is hard to find out. This decreases the morbid portion screening efficiency and delays finding of the morbid portion.
The same problems as described above also arise in the conventional X-ray CT. For example, when a to-be-detected morbid portion having a very low contrast is present near an internal structure such as a bone with a very high contrast, even a specialist in diagnostic reading can hardly perceive the morbid portion. Additionally, the conventional CT apparatus executes radiography while making a patient pass through a large special rotation mechanism called a gantry incorporating X-ray detection elements and an X-ray source. Since the arrangement itself is different from general radiographic apparatuses, there is a possibility that the CT apparatus is located in another room. This decreases the radiography efficiency.